None of the existing models of the respiratory control system adequately explain the precise relationship between the increase of ventilation and that of metabolism; nor do they explain the patterns of ventilation at the onset or offset of exercise. The proposed studies are based on the hypothesis that neural input to the respiratory control system and neural mechanisms within the system are more important than traditionally believed. The proposed studies use the neural output of the respiratory control system to represent ventilation, thereby allowing the use of an "open loop" experimental condition which avoids the negative chemical feedback associated with changes in ventilation. The first step has been to develop methods of handling phrenic nerve and respiratory muscle activity so that an objective and repeatable neural equivalent of tidal volume, a neural representation of central respiratory drive, is obtained. The phrenic output is used in paralyzed ventilated animals to study the effect of various neural inputs and mechanisms on central respiratory drive. Especially to be studied are (1) the apparent ability of active breathing to be self-stimulating and (2) the effect of timing of brief stimuli on expiratory as well as inspiratory respiratory drive. The findings should lead to a better understanding of control mechanisms during both resting and exertional breathing.